The present invention relates to black plate-shaped ferrite composite particles with magnetoplumbite structure and a magnetic recording medium using the black plate-shaped ferrite composite particles with magnetoplumbite structure. More particularly, the present invention relates to black plate-shaped ferrite composite particles with magnetoplumbite structure which are excellent in dispersibility in a vehicle due to less amount of carbon black fallen-off from the surface of each black plate-shaped ferrite composite particles with magnetoplumbite structure, and have a high blackness and a low volume resistivity, and a magnetic recording medium capable of not only showing a low light transmittance and a low surface resistivity even when the amount of carbon black fine particles added to a magnetic recording layer thereof is as small as possible, but also having a smooth surface, by using the black plate-shaped ferrite composite particles with magnetoplumbite structure.
With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly desired to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Especially, video tapes have recently been desired more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
In order to enhance output characteristics of magnetic recording media, especially an S/N ratio thereof with respect to signals having a short wavelength, there have been demanded fineness of magnetic particles, reduction in thickness of a magnetic recording layer, high dispersibility of magnetic particles and surface smoothness of a magnetic coating film.
In general, as magnetic particles having a high coercive force, there are known magnetic metal particles containing iron as a main component, plate-shaped ferrite particles with magnetoplumbite structure, or the like.
Since the above-mentioned plate-shaped ferrite particles with magnetoplumbite structure are a stable oxide compound, there have been a large demand thereof especially in the field of data tapes which are required to retain recorded information for a long period of time.
However, it is also known that due to the fact that the plate-shaped ferrite particles with magnetoplumbite structure are a stable oxide compound, electrons therewithin are less movable, thereby exhibiting a volume resistivity as high as not less than 1.0xc3x97108 xcexa9xc2x7cm. When such plate-shaped ferrite particles with magnetoplumbite structure are used as magnetic particles for magnetic recording media, there arises such a disadvantage that the surface resistivity of the obtained magnetic recording media becomes too high, especially more than 1.0xc3x971012 xcexa9/sq.
The magnetic recording media having such a high surface resistivity value cause problems such as attachment of cut chips, dusts or the like onto the surface thereof during the production process or upon use due to the increase in electrostatic charge amount, resulting in frequently generating drop-out. Therefore, in order to produce magnetic recording media whose surface resistivity value is reduced to not more than 1010 xcexa9/sq, it has been strongly demanded to provide plate-shaped ferrite particles with magnetoplumbite structure which have a volume resistivity value as low as possible.
On the other hand, at the present time, the end position of a magnetic recording medium such as magnetic tapes has been detected by sensing a high light transmittance portion of the magnetic recording medium by means of a video deck. In the case where the particle size of magnetic particles dispersed in the magnetic recording layer become finer and the thickness of the magnetic recording medium is reduced in order to meet the requirement for high performance of the magnetic recording medium as described hereinbefore, the magnetic recording medium shows a high light transmittance as a whole, so that it has been difficult to detect the end position thereof by means of the video deck. In order to solve this problem, carbon black fine particles have been added to the magnetic recording layer in an amount of usually about 6 to 12 parts by weight based on 100 parts by weight of the magnetic particles, thereby reducing the light transmittance of the magnetic recording medium. For this reason, in current videotapes, it is indispensable to add carbon black fine particles, etc., to the magnetic recording layer thereof.
When a large amount of carbon black fine particles are added to the magnetic recording layer, the obtained magnetic recording media can show not only a low light transmittance but also a low surface resistivity value, because the carbon black fine particles are conductive particles. However, since the carbon black fine particles are fine particles having an average particle size as small as about 0.002 to about 0.05 xcexcm, and have a large BET specific surface area and a low solvent-wettability, it is difficult to disperse these particles in vehicles. For this reason, it also becomes difficult to produce magnetic recording media having a smooth surface. In addition, since the addition of the large amount of such carbon black fine particles becomes to increase in amount of non-magnetic components in the magnetic recording layer, the obtained magnetic recording media is deteriorated in signal recording properties, thereby obstructing not only high-density recording but also reduction in thickness of the magnetic recording layer.
Consequently, it has been required to provide plate-shaped ferrite particles with magnetoplumbite structure having an excellent blackness and a low volume resistivity value in order to obtain magnetic recording media which can exhibit a sufficiently low surface resistivity value and a sufficiently low light transmittance even when the amount of carbon black fine particles added to the magnetic recording layer is reduced to a level as low as possible, especially less than 6 parts by weight based on 100 parts by weight of magnetic particles.
Hitherto, as methods of reducing the surface resistivity value of magnetic recording media using the plate-shaped ferrite particles with magnetoplumbite structure, there are known a method of adding a large amount of carbon fine particles to the magnetic recording layer, and a method of lowering the volume resistivity value of the magnetic particles per se, as described hereinabove. As the method of lowering the volume resistivity value, there are known a method of incorporating Fe2+ into plate-shaped hexagonal ferrite particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 62-154228(1987) and 2-208821(1990), etc.), a method of depositing 2 to 20% by weight of carbon onto the surfaces of particles by heat-treating plate-shaped hexagonal ferrite particles at a temperature of 100 to 450xc2x0 C. by a hydrogen gas stream and then treating the reduced particles under a carbon dioxide gas stream (Japanese Patent Application Laid-Open (KOKAI) No. 4-157615(1992), etc.), or the like.
At the present time, it has been most strongly demanded to provide plate-shaped ferrite particles with magnetoplumbite structure, which show an excellent blackness and a low volume resistivity value. However, such plate-shaped ferrite particles with magnetoplumbite structure, which can satisfy these properties have not been obtained yet.
That is, the above-mentioned Fe2+-containing plate-shaped hexagonal ferrite particles show a low volume resistivity value. However, as shown in Comparative Examples hereinafter, since the plate-shaped hexagonal ferrite particles are unsatisfactory in blackness, magnetic recording media obtained by using these particles cannot show a sufficiently low light transmittance. In addition, because of the inclusion of Fe2+, these particles are deteriorated in dispersibility in vehicles, so that it is difficult to obtain a magnetic recording layer having a smooth surface. Further, the plate-shaped hexagonal ferrite particles are deteriorated in oxidation stability, and tends to suffer from change in coercive force value with the passage of time due to the fact that Fe2+ contained therein is readily oxidized into Fe3+.
The above-mentioned carbon-deposited plate-shaped hexagonal ferrite particles exhibit an excellent blackness and a low volume resistivity value. However, because of the production process including heat-treating the plate-shaped hexagonal ferrite particles under a hydrogen gas stream, the obtained particles inevitably contain Fe2+ as shown in Comparative Examples hereinafter. Accordingly, the carbon-deposited plate-shaped hexagonal ferrite particles also have the same defects as described above, i.e., suffer from change in coercive force value with the passage of time. In addition, since the adhesion force of carbon particles onto the surfaces of the plate-shaped hexagonal ferrite particles is weak, carbon particles tend to be desorbed or fallen-off therefrom when the particles are dispersed in vehicles. As a result, the dispersibility of the particles in vehicles is deteriorated, so that the obtained magnetic recording media show a large light transmittance and are unsatisfactory in surface smoothness as shown in Comparative Examples hereinafter.
As a result of the present inventors"" earnest studies, it has been found that by forming on a surface of each plate-shaped ferrite particle with magnetoplumbite structure a coating layer comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtainable from an alkoxysilane compound,
(2) polysiloxanes or modified polysiloxanes, and
(3) fluoroalkyl organosilane compounds obtainable from a fluoroalkylsilane compound, and
forming on the surface of the organosilicon coating layer a carbon black coating in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of said plate-shaped ferrite particles with magnetoplumbite structure.
The thus obtained black plate-shaped ferrite composite particles with magnetoplumbite structure, having an average particle size of 0.01 to 0.2 xcexcm, are useful as magnetic particles for high-density magnetic recording media, because the black plate-shaped ferrite composite particles with magnetoplumbite structure can exhibit a high blackness and a low volume resistivity value without containing almost Fe2+, and have an excellent dispersibility, thereby enabling the production of such magnetic recording media which show a low light transmittance and a low surface resistivity value even when the amount of carbon black fine particles added to the magnetic recording layer thereof is reduced to a level as low as possible, and which are excellent in surface smoothness. The present invention has been attained based on the finding.
It is an object of the present invention to provide plate-shaped ferrite particles with magnetoplumbite structure, which can exhibit a high blackness and a low volume resistivity value without containing Fe2+ thereinto.
It is another object of the present invention to provide a magnetic recording medium capable of showing a low light transmittance, a low surface resistivity value and an excellent surface smoothness.
To accomplish the aims, in a first aspect of the present invention, there are provided black plate-shaped ferrite composite particles with magnetoplumbite structure, having an average particle size of 0.01 to 0.2 xcexcm, comprising:
plate-shaped ferrite particles with magnetoplumbite structure;
a coating layer formed on surface of the plate-shaped ferrite particles with magnetoplumbite structure, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtainable from an alkoxysilane compounds,
(2) polysiloxanes or modified polysiloxanes, and
(3) fluoroalkyl organosilane compounds obtainable from a fluoroalkylsilane compounds; and
a carbon black coating formed on the coating layer comprising the organosilicon compound, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure.
In a second aspect of the present invention, there are black plate-shaped ferrite composite particles with magnetoplumbite structure, having an average particle size of 0.01 to 0.2 xcexcm, comprising:
plate-shaped ferrite particles with magnetoplumbite structure;
a coating formed on at least part of the surface of the plate-shaped ferrite particles with magnetoplumbite structure, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 20% by weight, calculated as Al or SiO2, based on the total weight of the coated plate-shaped ferrite particles with magnetoplumbite structure;
an organosilicon coating layer formed on the surface of the oxide or hydroxide coating, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtainable from an alkoxysilane compounds,
(2) polysiloxanes or modified polysiloxanes, and
(3) fluoroalkyl organosilane compounds obtainable from a fluoroalkylsilane compounds; and
a carbon black coating formed on the organosilicon coating layer comprising the organosilicon compound, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure.
In a third aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film; and
a magnetic recording layer comprising a binder resin and black plate-shaped ferrite composite particles with magnetoplumbite structure, having an average particle size of 0.01 to 0.2 xcexcm, comprising:
plate-shaped ferrite particles with magnetoplumbite structure;
an organosilicon coating layer formed on surface of the plate-shaped ferrite particles with magnetoplumbite structure, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtainable from an alkoxysilane compounds,
(2) polysiloxanes or modified polysiloxanes, and
(3) fluoroalkyl organosilane compounds obtainable from a fluoroalkylsilane compounds; and
a carbon black coating formed on the organosilicon coating layer, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure.
In a fourth aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film; and
a magnetic recording layer comprising a binder resin and black plate-shaped ferrite composite particles with magnetoplumbite structure, having.an average particle size of 0.01 to 0.2 xcexcm, comprising:
plate-shaped ferrite particles with magnetoplumbite structure;
an oxide or hydroxide coating formed on at least part of the surface of the plate-shaped ferrite particles with magnetoplumbite structure, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 20% by weight, calculated as Al or SiO2, based on the total weight of the coated plate-shaped ferrite particles with magnetoplumbite structure;
an organosilicon coating layer formed on the surface of the oxide or hydroxide coating, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtainable from an alkoxysilane compounds,
(2) polysiloxanes or modified polysiloxanes, and
(3) fluoroalkyl organosilane compounds obtainable from a fluoroalkylsilane compounds; and
a carbon black coating formed on the organosilicon coating layer, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure.
First, the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention which are useful as magnetic particles for magnetic recording media, are explained.
The black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, have an average particle size of 0.01 to 0.2 xcexcm, and comprise plate-shaped ferrite particles with magnetoplumbite structure as core particles, a coating layer comprising an organosilicon compound which is formed on the surface of each plate-shaped ferrite particles with magnetoplumbite structure, and carbon black coat which are formed on the coating layer comprising the organosilicon compound.
As the plate-shaped ferrite particles with magnetoplumbite structure which are used as core particles in the present invention, there may be exemplified plate-shaped ferrite particles with magnetoplumbite structure, which contain Ba, Sr or both Ba and Sr; plate-shaped ferrite particles with magnetoplumbite structure which further contain, in addition to Ba and/or Sr, at least one coercive force-reducing agent selected from the group consisting of divalent or tetravalent metals such as Co, Ni, Zn, Mn, Mg, Ti, Nb, Cu, Zr, Mo and Sn; or the like.
The above-mentioned plate-shaped ferrite particles with magnetoplumbite structure used in the present invention, has a composition represented by the general formula:
AOxc2x7n{(Fe1xe2x88x92aMa)2O3}
wherein A is Ba, Sr or Baxe2x80x94Sr; M is at least one element selected from the group consisting of Co, Ni, Zn, Mn, Mg, Ti, Nb, Cu, Zr, Mo and Sn ; n is a number of 5.5 to 6.5; and a is a number of 0 to 0.5.
The average particle diameter (average plate diameter) of the plate-shaped ferrite particles with magnetoplumbite structure as core particles used in the present invention is usually 0.009 to 0.18 xcexcm, preferably 0.019 to 0.18 xcexcm, more preferably 0.027 to 0.18 xcexcm.
When the average particle diameter of the plate-shaped ferrite particles with magnetoplumbite structure is more than 0.18 xcexcm, the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure also may become large particles. In the case where such large particles are used for forming a magnetic recording layer, the surface smoothness of the magnetic recording layer tends to be deteriorated. On the other hand, when the average particle size is less than 0.009 xcexcm, the intermolecular force between the particles may be increased due to the reduction in particle size, so that agglomeration of the particles tends to be caused. Therefore, it becomes difficult to uniformly coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with the organosilicon compound, and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds.
The average thickness of the plate-shaped ferrite particles with magnetoplumbite structure as core particles is preferably 0.0005 to 0.045 xcexcm, more preferably 0.0009 to 0.045 xcexcm, still more preferably 0.0014 to 0.045.
When the average thickness of the plate-shaped ferrite particles with magnetoplumbite structure is more than 0.045 xcexcm, the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure also may become large particles. In the case where such large particles are used for forming a magnetic recording layer, the surface smoothness of the magnetic recording layer may tend to be deteriorated. On the other hand, when the average thickness is less than 0.0005 xcexcm, the intermolecular force between the particles may be increased due to the reduction in particle size, so that agglomeration of the particles may tend to be caused. Therefore, it may become difficult to uniformly coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with the organosilicon compound, and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds.
The BET specific surface area of the plate-shaped ferrite particles with magnetoplumbite structure as core particles is preferably 30 to 200 m2/g, more preferably 35 to 150 m2/g, still more preferably 38 to 100 m2/.
When the BET specific surface area of the plate-shaped ferrite particles with magnetoplumbite structure is less than 30 m2/g, the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure also may become large particles. In the case where such large particles are used for forming a magnetic recording layer, the surface smoothness of the magnetic recording layer may tend to be deteriorated. On the other hand, when the BET specific surface area is more than 200 m2/g, the intermolecular force between the particles may be increased due to the reduction in particle size, so that agglomeration of the particles may tend to be caused. Therefore, it may become difficult to uniformly coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with the organosilicon compound, and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon.compounds.
The plate-shaped ferrite particles with magnetoplumbite structure used in the present invention have a plate ratio (ratio of average particle size to average thickness; hereinafter referred to merely as xe2x80x9cplate ratioxe2x80x9d) of preferably 2.0:1 to 20.0:1, more preferably 2.5:1 to 15.0:1, still more preferably 3.0:1 to 10.0:1. When the plate ratio is more than 20.0:1, the stacking between particles may be frequently caused, so that it may become difficult to uniformly coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with the organosilicon compound, and then uniformly form the carbon black coat thereon. On the other hand, when the plate ratio is less than 2.0:1, the film strength of the obtained magnetic recording media may be deteriorated.
The geometrical standard deviation value of the particle diameter (plate diameter) of the plate-shaped ferrite particles with magnetoplumbite structure as core particles used in the present invention is preferably not more than 1.7, more preferably not more than 1.6. When the geometrical standard deviation value thereof is more than 1.7, coarse particles may be contained therein, so that the plate-shaped ferrite particles with magnetoplumbite structure may be inhibited from being uniformly dispersed. Therefore, it also may become difficult to uniformly coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with the organosilicon compounds and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds. The lower limit of the geometrical standard deviation value is 1.01. It is industrially difficult to obtain particles having a geometrical standard deviation value of less than 1.01.
The blackness of the plate-shaped ferrite particles with magnetoplumbite structure as core particles used in the present invention, is usually more than 18.0 when represented by L*value, and the upper limit thereof is preferably 34.0, more preferably 33.0 when represented by L*value. When the L*value exceeds 34.0, the lightness of the particles may be high, so that it is difficult to obtain black plate-shaped ferrite composite particles with magnetoplumbite structure having a sufficient blackness.
The volume resistivity of the plate-shaped ferrite particles with magnetoplumbite structure as core particles used in the present invention is usually not less than 1.0xc3x97108 xcexa9xc2x7cm. The upper limit thereof is usually about 1.0xc3x971010 xcexa9xc2x7cm.
As to the magnetic properties of the plate-shaped ferrite particles with magnetoplumbite structure as core particles used in the present invention, the coercive force value thereof is preferably 500 to 4,000 Oe, more preferably 650 to 4,000 Oe, the saturation magnetization value is preferably 40 to 70 emu/g, more preferably 45 to 70 emu/g.
The particle shape and particle size of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention are considerably varied depending upon those of plate-shaped ferrite particles with magnetoplumbite structure as core particles. The black plate-shaped ferrite composite particles with magnetoplumbite structure have a similar particle shape to that of the plate-shaped ferrite particles with magnetoplumbite structure as core particle, and a slightly larger particle size than that of the plate-shaped ferrite particles with magnetoplumbite structure as core particles.
More specifically, the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, have an average particle diameter (average plate diameter) of usually 0.01 to 0.2 xcexcm.
When the average particle size of the black plate-shaped ferrite composite particles with magnetoplumbite structure is more than 0.2 xcexcm, the black plate-shaped ferrite composite particles with magnetoplumbite structure may become coarse and as a result, the obtained coating film may become to be deteriorated in surface smooth. On the other hand, when the average particle size thereof is less than 0.01 xcexcm, the black plate-shaped ferrite composite particles with magnetoplumbite structure tends to be agglomerated by the increase of intermolecular force due to the reduction in particle size, thereby deteriorating the dispersibility in a vehicle upon production of the magnetic coating composition. In the consideration of the surface smoothness of the obtained coating film and the dispersibility in vehicles upon the production of magnetic coating compositions, the average particle size of the black plate-shaped ferrite composite particles with magnetoplumbite structure is preferably 0.02 to 0.2 xcexcm, more preferably 0.03 to 0.2 xcexcm.
The black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, have a thickness of usually 0.0005 to 0.05 xcexcm.
When the average thickness of the black plate-shaped ferrite composite particles with magnetoplumbite structure is more than 0.05 xcexcm, the black plate-shaped ferrite composite particles with magnetoplumbite structure may become coarse and as a result, the obtained coating film may become to be deteriorated in surface smooth. On the other hand, when the average thickness thereof is less than 0.0005 xcexcm, the black plate-shaped ferrite composite particles with magnetoplumbite structure may tend to be agglomerated by the increase of intermolecular force due to the reduction in particle size, thereby deteriorating the dispersibility in a vehicle upon production of the magnetic coating composition. In the consideration of the surface smoothness of the obtained coating film and the dispersibility in vehicles upon the production of magnetic coating compositions, the average thickness of the black plate-shaped ferrite composite particles with magnetoplumbite structure is preferably 0.001 to 0.05 xcexcm, more preferably 0.0015 to 0.05 xcexcm.
The black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, have a plate ratio of usually 2.0:1 to 20.0:1.
When the plate ratio of the black plate-shaped ferrite composite particles with magnetoplumbite structure is more than 20.0:1, the black plate-shaped ferrite composite particles with magnetoplumbite structure may tend to be frequently stacking between particles. As a result, upon the production of a magnetic coating composition, there is tendency that the dispersibility of the particles in vehicle is deteriorated and the viscosity of the obtained magnetic coating composition is increased. On the other hand, when the average thickness thereof is less than 2.0:1, the strength of the coating film of the obtained magnetic recording medium is low. In the consideration of the film strength of the obtained magnetic recording media and the dispersibility in vehicles upon the production of magnetic coating compositions, the plate ratio is preferably 2.5:1 to 15.0:1, more preferably 3.0:1 to 10.0:1.
The BET specific surface area of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, is preferably 30 to 200 m2/g, more preferably 35 to 150 m2/g, still more preferably 38 to 100 m2/g. When the BET specific surface area thereof is less than 30 m2/g, the black plate-shaped ferrite composite particles with magnetoplumbite structure may become coarse, and the sintering between the black plate-shaped ferrite composite particles with magnetoplumbite structure may be caused, thereby deteriorating the surface smooth of the magnetic recording layer. On the other hand, when the BET specific surface area is more than 200 m2/g, the black plate-shaped ferrite composite particles with magnetoplumbite structure may tend to be agglomerated together by the increase in intermolecular force due to the reduction in particle size, thereby deteriorating the dispersibility in the vehicle upon production of the magnetic coating composition.
The geometrical standard deviation value of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention is preferably not more than 1.7. When the geometrical standard deviation value thereof is more than 1.7, the surface smooth of the magnetic recording layer of the magnetic recording medium may be likely to be deteriorated due to the existence of coarse particles therein. With the consideration of the surface smooth of the magnetic recording layer, the geometrical standard deviation value thereof is more preferably not more than 1.6, still more preferably not more than 1.5. In the consideration of the industrial productivity, the lower limit of the geometrical standard deviation value thereof is preferably 1.01. It is industrially difficult to obtain such particles having a geometrical standard deviation of less than 1.01.
The volume resistivity of the black plate-shaped ferrite composite particles with magnetoplumbite structure is preferably less than 1.0xc3x97108 xcexa9xc2x7cm, more preferably 1.0xc3x97105 to 5.0xc3x97107 xcexa9xc2x7cm, still more preferably 1.0xc3x97105 to 1.0xc3x97107 xcexa9xc2x7cm. When the volume resistivity of the black plate-shaped ferrite composite particles with magnetoplumbite structure is not less than 1.0xc3x97108 xcexa9xc2x7cm, it may be difficult to sufficiently reduce the surface resistivity of the obtained magnetic recording medium.
As to the blackness of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, the upper limit of the blackness thereof is usually 27.0, preferably 26.0, more preferably 25.0 when represented by L*value. When the L*value thereof is more than 27.0, the lightness of the black plate-shaped ferrite composite particles with magnetoplumbite structure becomes high, so that the blackness of the black plate-shaped ferrite composite particles with magnetoplumbite structure is insufficient. The lower limit of the blackness thereof is 15 when represented by L*value.
The percentage of desorption of carbon black from the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, is preferably not more than 20%, more preferably not more than 10%. When the desorption percentage of the carbon black is more than 20%, the desorbed carbon black may tend to hinder the black plate-shaped ferrite composite particles with magnetoplumbite structure from being uniformly dispersed in the vehicle upon production of the magnetic coating composition.
The black plate-shaped ferrite composite particles with magnetoplumbite structure, have a ratio of Fe2+ to Fe3+ (hereinafter referred to merely as xe2x80x9cFe2+/Fe3+ ratioxe2x80x9d) of usually not more than 0.01:1, preferably not more than 0.005:1. When the Fe2+/Fe3+ ratio is more than 0.01:1, since Fe2+ tends to be oxidized into Fe3+, the coercive force value of the black plate-shaped ferrite composite particles with magnetoplumbite structure tends to vary with the passage of time. The lower limit of the Fe2+/Fe3+ is zero (0).
As the magnetic properties of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, the coercive force of the black plate-shaped ferrite composite particles with magnetoplumbite structure is preferably 500 to 4,000 Oe, more preferably 650 to 4,000 Oe, the saturation magnetization thereof is preferably 40 to 70 emu/g, more preferably 45 to 70 emu/g.
The percentage of change in coercive force value of the black plate-shaped ferrite composite particles with magnetoplumbite structure is preferably not more than 3%, more preferably not more than 2%.
The coating layer formed on the surface of the core particle comprises at least one organosilicon compound selected from the group consisting of (1) organosilane compounds obtainable from alkoxysilane compounds; (2) polysiloxanes, or modified polysiloxanes selected from the group consisting of (2-A) polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds (hereinafter referred to merely as xe2x80x9cmodified polysiloxanesxe2x80x9d), and (2-B) polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group (hereinafter referred to merely as xe2x80x9cterminal-modified polysiloxanesxe2x80x9d); and (3) fluoroalkyl organosilane compounds obtainable from fluoroalkylsilane compounds.
The organosilane compounds (1) may be produced by drying or heat-treating alkoxysilane compounds represented by the formula (I):
R1aSiX4xe2x88x92axe2x80x83xe2x80x83(I)
wherein R1 is C6H5xe2x80x94, (CH3)2CHCH2xe2x80x94 or n-CbH2b+13 (wherein b is an integer of 1 to 18); X is CH3Oxe2x80x94 or C2H5Oxe2x80x94; and a is an integer of 0 to 3.
The drying or heat-treatment of the alkoxysilane compounds may be conducted, for example, at a temperature of usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.
Specific examples of the alkoxysilane compounds may include methyl triethoxysilane, dimethyl diethoxysilane, phenyl triethyoxysilane, diphenyl diethoxysilane, methyl trimethoxysilane, dimethyl dimethoxysilane, phenyl trimethoxysilane, diphenyl dimethoxysilane, isobutyl trimethoxysilane, decyl trimethoxysilane or the like. Among these alkoxysilane compounds, in view of the desorption percentage and the coating effect of carbon black, methyl triethoxysilane, phenyl triethyoxysilane, methyl trimethoxysilane, dimethyl dimethoxysilane and isobutyl trimethoxysilane are preferred, and methyl triethoxysilane and methyl trimethoxysilane are more preferred.
As the polysiloxanes (2), there may be used those compounds represented by the formula (II): 
wherein R2 is Hxe2x80x94 or CH3xe2x80x94, and d is an integer of 15 to 450.
Among these polysiloxanes, in view of the desorption percentage and the coating effect of carbon black, polysiloxanes having methyl hydrogen siloxane units are preferred.
As the modified polysiloxanes (2-A), there may be used:
(a) polysiloxanes modified with polyethers represented by the formula (III): 
wherein R3 is xe2x80x94(xe2x80x94CH2xe2x80x94)hxe2x80x94; R4 is xe2x80x94(xe2x80x94CH2xe2x80x94)ixe2x80x94CH3; R5 is xe2x80x94OH, xe2x80x94COOH, xe2x80x94CHxe2x95x90CH2, xe2x80x94C(CH3)xe2x95x90CH2 or xe2x80x94(xe2x80x94CH2xe2x80x94)jxe2x80x94CH3; R6 is xe2x80x94(xe2x80x94CH2xe2x80x94)kxe2x80x94CH3; g and h are an integer of 1 to 15; i, j and k are an integer of 0 to 15; e is an integer of 1 to 50; and f is an integer of 1 to 300;
(b) polysiloxanes modified with polyesters represented by the formula (IV): 
wherein R7, R8 and R9 are xe2x80x94(xe2x80x94CH2xe2x80x94)qxe2x80x94 and may be the same or different; R10 is xe2x80x94OH, xe2x80x94COOH, xe2x80x94CHxe2x95x90CH2, xe2x80x94C(CH3)xe2x95x90CH2 or xe2x80x94(xe2x80x94CH2xe2x80x94)rxe2x80x94CH3; R11 is xe2x80x94(xe2x80x94CH2xe2x80x94)sxe2x80x94CH3; n and q are an integer of 1 to 15; r and s are an integer of 0 to 15; exe2x80x2 is an integer of 1 to 50; and fxe2x80x2 is an integer of 1 to 300;
(c) polysiloxanes modified with epoxy compounds represented by the formula (V): 
wherein R12 is xe2x80x94(xe2x80x94CH2xe2x80x94)vxe2x80x94; v is an integer of 1 to 15; t is an integer of 1 to 50; and u is an integer of 1 to 300; or a mixture thereof.
Among these modified polysiloxanes (2-A), in view of the desorption percentage and the coating effect of carbon black, the polysiloxanes modified with the polyethers represented by the formula (III), are preferred.
As the terminal-modified polysiloxanes (2-B), there may be used those represented by the formula (VI): 
wherein R13 and R14 are xe2x80x94OH, R16OH or R17COOH and may be then same or different; R15 is xe2x80x94CH3 or xe2x80x94C6H5; R16 and R17 are xe2x80x94(xe2x80x94CH2xe2x80x94)yxe2x80x94; y is an integer of 1 to 15; w is an integer of 1 to 200; and x is an integer of 0 to 100.
Among these terminal-modified polysiloxanes, in view of the desorption percentage and the coating effect of carbon black, the polysiloxanes whose terminals are modified with carboxylic acid groups are preferred.
The fluoroalkyl organosilane compounds (3) may be produced by drying or heat-treating fluoroalkylsilane compounds represented by the formula (VII):
CF3(CF2)zCH2CH2(R18)axe2x80x2SiX4xe2x88x92axe2x80x2xe2x80x83xe2x80x83(VII)
wherein R18 is CH3xe2x80x94, C2H5xe2x80x94, CH3Oxe2x80x94 or C2H5Oxe2x80x94; X is CH3Oxe2x80x94 or C2H5Oxe2x80x94; and z is an integer of 0 to 15; and axe2x80x2 is an integer of 0 to 3.
The drying or the heat-treatment of the fluoroalkylsilane compounds may be conducted, for example, at a temperature of usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.
Specific examples of the fluoroalkylsilane compounds may include trifluoropropyl trimethoxysilane, tridecafluorooctyl trimethoxysilane, heptadecafluorodecyl trimethoxysilane, heptadecafluorodecylmethyl dimethoxysilane, trifluoropropyl triethoxysilane, tridecafluorooctyl triethoxysilane, heptadecafluorodecyl triethoxysilane, heptadecafluorodecylmethyl diethoxysilane or the like. Among these fluoroalkylsilane compounds, in view of the desorption percentage and the coating effect of carbon black, trifluoropropyl trimethoxysilane, tridecafluorooctyl trimethoxysilane and heptadecafluorodecyl trimethoxysilane are preferred, and trifluoropropyl trimethoxysilane and tridecafluorooctyl trimethoxysilane are more preferred.
The coating amount of the organosilicon compounds is usually 0.02 to 5.0% by weight, preferably 0.03 to 4.0% by weight, more preferably 0.05 to 3.0% by weight (calculated as Si) based on the weight of the plate-shaped ferrite particles with magnetoplumbite structure coated with the organosilicon compounds.
When the coating amount of the organosilicon compounds is less than 0.02% by weight, it becomes difficult to form the carbon black coat on the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure in such an amount enough to improve the blackness of the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure.
On the other hand, when the coating amount of the organosilicon compounds is more than 5.0% by weight, a sufficient amount of the carbon black coat can be formed on the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure. However, the use of such unnecessarily large amount of the organosilicon compounds is meaningless because the effect of enhancing the blackness of the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure is already saturated.
As the carbon black fine particles used in the present invention, there may be exemplified commercially available carbon blacks such as furnace black, channel black or the like. Specific examples of the commercially available carbon blacks usable in the present invention, may include #3050, #3150, #3250, #3750, #3950, MA-100, MA7, #1000, #2400B, #30, MA8, MA11, #50, #52, #45, #2200B, MA600, etc. (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM, etc. (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860, Raven 1000, Raven 1190 ULTRA, etc. (tradename, produced by COLOMBIAN CHEMICALS COMPANY), Ketchen black EC, Ketchen black EC600JD, etc. (tradename, produced by KETCHEN INTERNATIONAL CO., LTD.), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, VULCAN XC72, REGAL 660, REGAL 400, etc. (tradename, produced by CABOTT SPECIALTY CHEMICALS INK CO., LTD.), or the like. In view of the compatibility with the organosilicon compounds, MA-100, MA7, #1000, #2400B, #30, BLACK PEARLS-L and BLACK PEARLS 4630 are preferred.
The lower limit of the average particle size of the carbon black fine particles used is usually 0.002 xcexcm, preferably 0.0025 xcexcm, and upper limit thereof is usually 0.05 xcexcm, preferably 0.035 xcexcm. When the average particle size of the carbon black fine particles used is less than 0.002 xcexcm, the carbon black fine particles used are too fine to be well handled.
On the other hand, when the average particle size thereof is more than 0.05 xcexcm since the particle size of the carbon black fine particles used is much larger, it is necessary to apply a larger mechanical shear force for forming the uniform carbon black coat on the coating layer composed of the organosilicon compounds, thereby rendering the coating process industrially disadvantageous.
The amount of the carbon black coat is 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure as core particles.
When the amount of the carbon black coat formed is less than 0.5 part by weight, the amount of the carbon black may be insufficient, so that it becomes difficult to obtain black plate-shaped ferrite composite particles with magnetoplumbite structure having a sufficient blackness and lower volume resistivity.
On the other hand, when the amount of the carbon black coat is more than 10 parts by weight, the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure can show a sufficient blackness and volume resistivity. However, since the amount of the carbon black is considerably large, the carbon black tend to be desorbed from the coating layer composed of the organosilicon compound. As a result, the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure tend to be deteriorated in dispersibility in a vehicle upon the production of magnetic coating composition.
The thickness of carbon black coat formed is preferably not more than 0.04 xcexcm, more preferably not more than 0.03 xcexcm, still more preferably not more than 0.02 xcexcm The lower limit thereof is more preferably 0.0001 xcexcm.
At least a part of the surface of the plate-shaped ferrite particles with magnetoplumbite structure as a core particle used in the present invention may be coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon (hereinafter referred to as xe2x80x9chydroxides and/or oxides of aluminum and/or silicon coatxe2x80x9d). When the black plate-shaped ferrite composite particles with magnetoplumbite structure obtained by using as core particles the plate-shaped ferrite particles with magnetoplumbite structure which are coated with the hydroxides and/or oxides of aluminum and/or silicon, are dispersed in a vehicle, since the treated particles have an affinity with the binder resin, it is more easy to obtain a desired dispersibility.
The amount of the hydroxides and/or oxides of aluminum and/or silicon coat is usually not more than 20% by weight, preferably 0.01 to 20% by weight, more preferably 0.05 to 10% by weight (calculated as Al and/or SiO2) based on the total weight of the coated plate-shaped ferrite particles with magnetoplumbite structure. If it is less than 0.01% by weight (calculated as Al and/or SiO2) based on the total weight of the coated plate-shaped ferrite particles with magnetoplumbite structure, the dispersibility-improving effect by coating therewith may be insufficient. If the amount exceeds 20% by weight (calculated as Al and/or SiO2) based on the total weight of the coated plate-shaped ferrite particles with magnetoplumbite structure, the dispersibility-improving effect by coating therewith becomes saturated, so that it is meaningless to add a coating material more than necessary.
The black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention wherein the plate-shaped ferrite particles with magnetoplumbite structure which are coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon are used as core particles, are substantially identical in particle size, geometrical standard deviation, BET specific surface area value, volume resistivity value, blackness L*value, magnetic properties, desorption percentage of carbon black, Fe2+/Fe3+ and percentage of change in coercive force value with the passage of time, to those of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention which are uncoated with the hydroxide and/or oxide of aluminum and/or silicon.
Next, the magnetic recording medium according to the present invention is described.
The magnetic recording medium according to the present invention comprises:
a non-magnetic substrate; and
a magnetic recording layer formed on the non-magnetic substrate, comprising a binder resin and the black plate-shaped ferrite composite particles with magnetoplumbite structure.
As the non-magnetic base film, the following materials which are at present generally used for the production of a magnetic recording medium may be used: a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide and polyimide; foil and plate of a metal such as aluminum and stainless steel; and various kinds of paper. The thickness of the non-magnetic base film varies depending upon the material, but it is usually about 1.0 to 300 xcexcm, preferably 2.0 to 200 xcexcm.
In the case of a magnetic disc, polyethylene terephthalate is usually used as the non-magnetic base film, and the thickness thereof is usually 50 to 300 xcexcm preferably 60 to 200 xcexcm. In a magnetic tape, when polyethylene terephthalate is used as the non-magnetic base film, the thickness thereof is usually 3 to 100 xcexcm, preferably 4 to 20 xcexcm; when polyethylene naphthalate is used, the thickness thereof is usually 3 to 50 xcexcm, preferably 4 to 20 xcexcm; and when polyamide is used, the thickness thereof is usually 2 to 10 xcexcm, preferably 3 to 7 xcexcm.
As the binder resin used in the present invention, the following resins which are at present generally used for the production of a magnetic recording medium may be used: vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron radiation curing acryl urethane resin and mixtures thereof.
Each of these resin binders may contain a functional group such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94OPO2M2 and xe2x80x94NH2, wherein M represents H, Na or K. With the consideration of the dispersibility of the black plate-shaped ferrite composite particles with magnetoplumbite structure, a binder resin containing a functional group xe2x80x94COOH or xe2x80x94SO3M is preferable.
The thickness of the magnetic recording layer obtained by applying the magnetic coating composition on the surface of the non-magnetic substrate and dried, is usually in the range of 0.01 to 5.0 xcexcm. If the thickness is less than 0.01 xcexcm, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 xcexcm, it may be difficult to obtain desired signal recording property due to an influence of diamagnetism. The preferable thickness is in the range of 0.1 to 4.0 xcexcm.
The mixing amount of the black plate-shaped ferrite composite particles with magnetoplumbite structure to the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin.
When the amount of the black plate-shaped ferrite composite particles with magnetoplumbite structure blended is less than 5 parts by weight, the obtained magnetic coating composition contains a too small amount of the black plate-shaped ferrite composite particles with magnetoplumbite structure. As a result, when a coating film is produced from such a magnetic coating composition, it is not possible to obtain a coating film in which the black plate-shaped ferrite composite particles with magnetoplumbite structure are continuously dispersed, so that the surface smoothness and the strength of the coating film become unsatisfactory. On the other hand, when the amount of the black plate-shaped ferrite composite particles with magnetoplumbite structure blended is more than 2,000 parts by weight, the amount of the black plate-shaped ferrite composite particles with magnetoplumbite structure becomes too large relative to that of the binder resin, so that it is not possible to sufficiently disperse the black plate-shaped ferrite composite particles with magnetoplumbite structure in the magnetic coating composition. As a result, when a coating film is produced from such a magnetic coating composition, it is difficult to obtain a coating film having a sufficiently smooth surface. Further, since the black plate-shaped ferrite composite particles with magnetoplumbite structure cannot be sufficiently bound with each other by the binder resin, the obtained coating film tends to become brittle.
In the magnetic recording medium according to the present invention, the amount of carbon black fine particles added to the magnetic recording layer thereof can be reduced to usually less than 6 parts by weight, preferably less than 5 parts by weight, more preferably less than 3 parts by weight based on 100 parts by weight of the black plate-shaped ferrite composite particles with magnetoplumbite structure.
Further, in the case where the black plate-shaped ferrite composite particles with magnetoplumbite structure wherein the particle size thereof is large and a large amount of the carbon black coat can be formed onto the surface thereof, especially in an amount of 7 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure, are used as magnetic particles, it can be expected to omit the addition of the carbon black fine particles to the magnetic recording layer.
Incidentally, the magnetic recording layer may optionally contain a lubricant, an abrasive, an anti-static agent and other additives which are usually used for the production of magnetic recording media, in an amount of 0.1 to 50 parts by weight based on 100 parts of the binder resin.
In the magnetic recording medium according to the present invention, at least one non-magnetic undercoat layer comprising a binder resin and non-magnetic particles such as hematite particles or the like may be sandwiched between the non-magnetic base film and the magnetic coating film. As the binder resin used therein, resins which are at present generally used for the production of a magnetic recording medium may be used. The mixing ratio of the non-magnetic particles with the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin. The thickness of the non-magnetic undercoat layer obtained by applying a non-magnetic coating composition comprising the binder resin, an solvent and the non-magnetic particles, on the surface of the non-magnetic base film and drying, is usually 0.2 to 10.0 xcexcm, preferably 0.5 to 5.0 xcexcm. Further, it is possible to add a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium to the non-magnetic undercoat layer.
The magnetic recording medium having the non-magnetic undercoat layer according to the present invention, can be more improved in a light transmittance, a smooth surface and a strength of the coating film.
The magnetic recording medium according to the present invention has a coercive force of usually 500 to 4,000 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.82 to 0.95; a gloss (of the coating film) of usually 165 to 300%; a linear adsorption coefficient (of the coating film) of usually 1.20 to 10.0 xcexcmxe2x88x921; a surface roughness Ra (of the coating film) of usually not more than 12.0 nm; a surface resistivity of not more than 1.0xc3x971010 xcexa9/sq; and a Young""s modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160.
In case of using the black plate-shaped ferrite composite particles with magnetoplumbite structure as magnetic particles, wherein the plate-shaped ferrite particles with magnetoplumbite structure which are uncoated with the hydroxides and/or oxides of aluminum and/or silicon, are used as core particles, the magnetic recording medium according to the present invention has a coercive force of usually 500 to 4,000 Oe, preferably 650 to 4,000 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.82 to 0.95, preferably 0.83 to 0.95; a gloss (of the coating film) of usually 165 to 300%, preferably 170 to 300%; a linear adsorption coefficient (of the coating film) of usually 1.20 to 10.0 xcexcmxe2x88x921, preferably 1.25 to 10.0 xcexcmxe2x88x921; a surface roughness Ra (of the coating film) of usually not more than 12.0 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0 nm; a surface resistivity of usually not more than 1xc3x971010 xcexa9/sq, preferably not more than 7.5xc3x97109 xcexa9/sq, more preferably not more than 5.0xc3x97109 xcexa9/sq; and a Young""s modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 125 to 160.
In case of using the black plate-shaped ferrite composite particles with magnetoplumbite structure as magnetic particles, wherein the plate-shaped ferrite particles with magnetoplumbite structure which are coated with the hydroxides and/or oxides of aluminum and/or silicon are used as core particles, the magnetic recording medium according to the present invention has a coercive force of usually 500 to 4,000 Oe, preferably 650 to 4,000 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.82 to 0.95, preferably 0.83 to 0.95; a gloss (of the coating film) of usually 170 to 300%, preferably 175 to 300%; a linear adsorption coefficient (of the coating film) of usually 1.20 to 10.0 xcexcmxe2x88x921, preferably 1.25 to 10.0 xcexcmxe2x88x921; a surface roughness Ra (of the coating film) of usually not more than 11.5 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 9.5 nm, a surface resistivity of usually not more than 1.0xc3x971010 xcexa9/sq, preferably not more than 7.5xc3x97109 xcexa9/sq, more preferably not more than 5.0xc3x97109 xcexa9/sq; and a Young""s modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 125 to 160, preferably 126 to 160.
The black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention can be produced by the following method.
The coating of the plate-shaped ferrite particles with magnetoplumbite structure with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds, may be conducted (i) by mechanically mixing and stirring the plate-shaped ferrite particles with magnetoplumbite structure together with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds; or (ii) by mechanically mixing and stirring both the components together while spraying the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds onto the plate-shaped ferrite particles with magnetoplumbite structure. In these cases, substantially whole amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds added can be coated onto the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure.
In order to uniformly coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure as core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds, it is preferred that the plate-shaped ferrite particles with magnetoplumbite structure are preliminarily diaggregated by using a pulverizer.
As apparatus (a) for mixing and stirring the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds to form the coating layer thereof, and (b) for mixing and stirring carbon black fine particles with the particles whose surfaces are coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds to form the carbon black coat, there may be preferably used those apparatus capable of applying a shear force to the particles, more preferably those apparatuses capable of conducting the application of shear force, spaturate force and compressed force at the same time. In addition, by conducting the above mixing or stirring treatment (a) of the core particles together with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds, at least a part of the alkoxysilane compounds and the fluoroalkylsilane compounds coated on the core particles may be changed to the organosilane compounds and fluoroalkyl organosilane compounds, respectively.
As such apparatuses, there may be exemplified wheel-type kneaders, ball-type kneaders, blade-type kneaders, roll-type kneaders or the like. Among them, wheel-type kneaders are preferred.
Specific examples of the wheel-type kneaders may include an edge runner (equal to a mix muller, a Simpson mill or a sand mill), a multi-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller, or the like. Among them, an edge runner, a multi-mull, a Stotz mill, a wet pan mill and a ring muller are preferred, and an edge runner is more preferred.
Specific examples of the ball-type kneaders may include a vibrating mill or the like. Specific examples of the blade-type kneaders may include a Henschel mixer, a planetary mixer, a Nawter mixer or the like. Specific examples of the roll-type kneaders may include an extruder or the like.
In order to coat the surfaces of the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds as uniformly as possible, the conditions of the above mixing or stirring treatment may be appropriately controlled such that the linear load is usually 2 to 200 Kg/cm, preferably 10 to 150 Kg/cm, more preferably 15 to 100 Kg/cm; and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds added, is preferably 0.15 to 45 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure. When the amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds added is less than 0.15 part by weight, it may become difficult to form the carbon black coat in such an amount enough to improve the blackness and volume resistivity of the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure.
On the other hand, when the amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds added is more than 45 parts by weight, a sufficient amount of the carbon black coat can be formed on the surface of the coating, but it is meaningless because the blackness and volume resistivity of the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure cannot be further improved by using such an excess amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds added.
Next, the carbon black fine particles are added to the plate-shaped ferrite particles with magnetoplumbite structure coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds, and the resultant mixture is mixed and stirred to form the carbon black coat on the surfaces of the coating composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds added. In addition, by conducting the above mixing or stirring treatment (b) of the carbon black fine particles together with the plate-shaped ferrite particles with magnetoplumbite structure coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds, at least a part of the alkoxysilane compounds and the fluoroalkylsilane compounds coated on the plate-shaped ferrite particles with magnetoplumbite structure as the core particles may be changed to the organosilane compounds and fluoroalkyl organosilane compounds, respectively.
In the case where the alkoxysilane compounds and the fluoroalkylsilane compounds are used as the coating compound, after the carbon black coat is formed on the surface of the coating layer, the resultant composite particles may be dried or heat-treated, for example, at a temperature of usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.
It is preferred that the carbon black fine particles are added little by little and slowly, especially about 5 to 60 minutes.
In order to form carbon black coat onto the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds as uniformly as possible, the conditions of the above mixing or stirring treatment can be appropriately controlled such that the linear load is usually 2 to 200 Kg/cm, preferably 10 to 150 Kg/cm more preferably 15 to 100 Kg/cm; and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the carbon black fine particles added is preferably 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure. When the amount of the carbon black fine particles added is less than 0.5 part by weight, it may become difficult to form the carbon black coat in such an amount enough to improve the blackness and volume resistivity of the obtained black plate-shaped ferrite composite particles with magnetoplumbite structure. On the other hand, when the amount of the carbon black fine particles added is more than 10 parts by weight, a sufficient blackness and volume resistivity of the resultant black plate-shaped ferrite composite particles with magnetoplumbite structure can be obtained, but the carbon black tend to be desorbed from the surface of the coating layer because of too large amount of the carbon black, resulting in deteriorated dispersibility in the vehicle upon the production of the magnetic coating composition.
The plate-shaped ferrite particles with magnetoplumbite structure may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, if required, prior to mixing and stirring with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, the terminal-modified polysiloxanes or the fluoroalkylsilane compounds.
The coating of the hydroxides and/or oxides of aluminum and/or silicon may be conducted by adding an aluminum compound, a silicon compound or both the compounds to a water suspension in which the plate-shaped ferrite particles with magnetoplumbite structure are dispersed, followed by mixing and stirring, and further adjusting the pH value of the suspension, if required, thereby coating the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. The thus obtained plate-shaped ferrite particles with magnetoplumbite structure coated with the hydroxides and/or oxides of aluminum and/or silicon are then filtered out, washed with water, dried and pulverized. Further, the particles coated with the hydroxides and/or oxides of aluminum and/or silicon may be subjected to post-treatments such as deaeration treatment and compaction treatment, if required.
As the aluminum compounds, there may be exemplified aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride or aluminum nitrate, alkali aluminates such as sodium aluminate or the like.
The amount of the aluminum compound added is 0.01 to 20% by weight (calculated as Al) based on the weight of the plate-shaped ferrite particles with magnetoplumbite structure. When the amount of the aluminum compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with hydroxides and/or oxides of aluminum, thereby failing to achieve the improvement of the dispersibility in the vehicle upon the production of the magnetic coating composition. On the other hand, when the amount of the aluminum compound added is more than 20% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the aluminum compound.
As the silicon compounds, there may be exemplified #3 water glass, sodium orthosilicate, sodium metasilicate or the like.
The amount of the silicon compound added is 0.01 to 20% by weight (calculated as SiO2) based on the weight of the plate-shaped ferrite particles with magnetoplumbite structure. When the amount of the silicon compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure with hydroxides and/or oxides of silicon, thereby failing to achieve the improvement of the dispersibility in the vehicle upon the production of the magnetic coating composition. On the other hand, when the amount of the silicon compound added is more than 20% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the silicon compound.
In the case where both the aluminum and silicon compounds are used in combination for the coating, the total amount of the aluminum and silicon compounds added is preferably 0.01 to 20% by weight (calculated as a sum of Al and SiO2) based on the weight of the plate-shaped ferrite particles with magnetoplumbite structure.
The process of the magnetic recording medium according to the present invention is described as follows.
The magnetic recording medium according to the present invention can be produced by applying a magnetic coating composition containing the black plate-shaped ferrite composite particles with magnetoplumbite structure, a binder resin and a solvent, on the non-magnetic substrate, followed by drying, to form a magnetic recording layer thereon.
As the solvents, there may be used methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture of these solvents or the like.
The total amount of the solvent used is 65 to 1,000 parts by weight based on 100 parts by weight of the black plate-shaped ferrite composite particles with magnetoplumbite structure. When the amount of the solvent used is less than 65 parts by weight, the viscosity of the magnetic coating composition prepared therefrom becomes too high, thereby making it difficult to apply the magnetic coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous.
An point of the present invention lies in that the black plate-shaped ferrite composite particles with magnetoplumbite structure having an average particle size of 0.01 to 0.2 xcexcm, comprising as core particles the plate-shaped ferrite particles with magnetoplumbite structure which may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon; the organosilicon compounds coated on the surface of the plate-shaped ferrite particles with magnetoplumbite structure; and the carbon black coat formed on the surface of the coating layer comprising the organosilicon compounds, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the plate-shaped ferrite particles with magnetoplumbite structure, can show excellent in dispersibility in vehicle due to less amount of carbon black fallen-off from the surface of each black plate-shaped ferrite particles with magnetoplumbite structure, and have a high blackness and a low volume resistivity.
The reason why the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention can show an excellent blackness, is considered such that since the carbon black coat is uniformly and densely formed on the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure, the color tone of the core particles is hidden behind the carbon black, so that an inherent color tone of carbon black can be exhibited.
The reason why the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention can show a low volume resistivity is considered as follows. That is, the carbon black coat having an excellent conductivity can be uniformly and densely formed onto the surface of each black plate-shaped ferrite composite particles with magnetoplumbite structure.
The reason why the amount of the carbon black desorbed or fallen-off from the surfaces of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, is small, is considered as follows.
In the case of using as organosilane compounds, the alkoxysilane compounds and the fluoroalkylsilane compounds, metalloxane bonds (xe2x89xa1Sixe2x80x94Oxe2x80x94M wherein M represents a metal atom contained in the plate-shaped ferrite particles with magnetoplumbite structure, such as Si, Al, Fe or the like) are formed between the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure and alkoxy groups contained in the organosilicon compounds onto which the carbon black coat is formed, thereby forming a stronger bond between the organosilicon compounds on which the carbon black coat is formed, and the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure.
In the case of using as organosilane compounds, polysiloxanes or modified polysiloxanes, functional groups contained in the polysiloxanes or modified polysiloxanes onto which the carbon black coat is formed, thereby forming a stronger bond between the polysiloxanes or modified polysiloxanes on which the carbon black coat is formed, and the surfaces of the plate-shaped ferrite particles with magnetoplumbite structure.
The reason why the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention can exhibit an excellent dispersibility in vehicles upon the production of magnetic coating compositions, is considered as follows. That is, due to the fact that a less amount of carbon black are desorbed or fallen-off from the surfaces of the black plate-shaped ferrite composite particles with magnetoplumbite structure, the black plate-shaped ferrite composite particles with magnetoplumbite structure are free from the deterioration in dispersibility due to the desorbed or fallen-off carbon black.
In addition, the black plate-shaped ferrite composite particles with magnetoplumbite structure which tend to be stacked together can be prevented from being contacted with each other. Further, since the carbon black coat is non-magnetic, the black plate-shaped ferrite composite particles with magnetoplumbite structure can be prevented from being magnetically aggregated together.
The magnetic recording medium according to the present invention which is obtained by using the above-mentioned black plate-shaped ferrite composite particles with magnetoplumbite structure as magnetic particles, can show a low light transmittance and a low surface resistivity even when the amount of carbon black fine particles added to the magnetic recording layer is as small as possible, and the magnetic recording layer thereof can exhibit an improved surface smoothness.
The reason why the magnetic recording medium can show a low light transmittance even when the amount of carbon black fine particles added to the magnetic recording layer is small, is considered as follows. That is, in the case of the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention, the carbon black can be uniformly and densely coated onto the surface of each plate-shaped ferrite particle with magnetoplumbite structure and, therefore, can be dispersed in the coating film in such a condition close to primary particles, whereby the carbon black can effectively exhibit their own functions.
The reason why the surface resistivity of the magnetic recording medium can be kept low even when the amount of carbon black fine particles added to the magnetic recording layer is small, is considered as follows. That is, due to the fact that the black plate-shaped ferrite composite particles with magnetoplumbite structure are uniformly dispersed in the coating film, the carbon black coat uniformly and densely formed onto the surfaces thereof are continuously contacted with each other.
The reason why the magnetic recording medium according to the present invention can show an excellent surface smoothness, is considered as follows. That is, in the present invention, since the amount of the carbon black fine particles added to the magnetic recording layer is reduced to as small a level as possible, the black plate-shaped ferrite composite particles with magnetoplumbite structure can maintain a good dispersibility in vehicle upon production of the magnetic coating composition without being adversely affected by the carbon black fine particles added. Further, on the above-mentioned reason, the black plate-shaped ferrite composite particles with magnetoplumbite structure themselves can exhibit an excellent dispersibility.
The black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention can exhibit a high blackness and a low volume resistivity value without containing Fe2+ thereinto, and are excellent in dispersibility in vehicles. Therefore, in the case where the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention are used as magnetic particles, it becomes possible to produce a magnetic recording medium which has a low light transmittance and a low surface resistivity value even when the amount of carbon black fine particles added to the magnetic recording layer thereof is reduced to a level as small as possible, and which can show an excellent surface smoothness. Accordingly, the black plate-shaped ferrite composite particles with magnetoplumbite structure according to the present invention are useful as magnetic particles for high-density magnetic recording media.
As described above, due to the fact that the black plate-shaped ferrite composite particles with magnetoplumbite structure show an excellent blackness and a low volume resistivity, the magnetic recording medium according to the present invention can exhibit a low light transmittance and a low surface resistivity. Further, since the amount of carbon black fine particles added to the magnetic recording layer is reduced to as small a level as possible and the dispersibility of the black plate-shaped ferrite composite particles with magnetoplumbite structure is enhanced, the magnetic recording medium can have a smooth surface. Therefore, the magnetic recording medium according to the present invention can be suitably used as those for high-density recording.
Meanwhile, in the magnetic recording medium according to the present invention, by using the black plate-shaped ferrite composite particles with magnetoplumbite structure as magnetic particles, the amount of carbon black fine particles added to the magnetic recording layer, which have an average particle size as fine as about 0.002 to about 0.05 xcexcm, a large BET specific surface area and a poor solvent-wettability, and are therefore, deteriorated in dispersibility in vehicles, can be reduced to as small a level as possible. Accordingly, the magnetic recording medium according to the present invention is advantageous from industrial and economical viewpoints, and is favorable from the standpoints of safety and hygiene.